According to modern historiography, around the year 1000, the northern Hungarian duchy with its capital in Nitra was divided into four parts which can be predicted with more or less certainty as direct descendants of tribes both within and beyond the Great Moravian Empire: Nitra, Hont, Váh, Borsod, and, in addition, in the very eastern part of today's Slovak Republic, there was a tribe which was not the part of Nitra Duchy. The consideration of tribes is important. A tribe speaks a dialect. However, the division of Slovakia was probably more complicated and on the basis of both genealogical and linguistic research we have to accept at least seven tribes and dialects in Slovakia around the year 1000. This division is supported also by the research of Proto-Slavic lexis in Old Slovak. The paper deals with hierarchical cluster analysis of this lexis and offers a solution to the genealogical problem of Slovak language. According to it, Old Slovak can be divided into eastern and west-central parts. The question naturally arises whether this approach can be regarded as methodologically correct.